1. Field of the Invention
This invention relates to heat resistant resin solutions, and, more precisely, heat resistant resin solutions of polymers obtained by reaction of a 1,2,3,4-butanetetracarboxylic acid (hereinafter simply referred to as BTC), an aromatic tricarboxylic acid anhydride and a diamine in the presence of a polyhydric alcohol, the polymer having a residual acid value of about 5 to 40% dissolved in aqueous ammonia and at least one common solvent selected from the group consisting of alcohols, polyhydric alcohols, polyhydric alcohol derivatives, ketones, ethers and esters, as well as to a method for preparation of such resin solutions.
2. Description of the Prior Art
Polyimide resins derived from BTC and a diamine are known as disclosed in U.S. Pat. No. 3,673,145 and Japanese Patent Publication No. 14503/72, and these are polymers having excellent heat resistance. For example, these polymers are useful as films or insulating varnishes. In these polyimide resins, however, the polycarboxylic acid (BTC) component to be reacted with the other diamine component is an aliphatic compound, and therefore, the heat resistance of the obtained polyimide resin, although being relatively high, is inferior to that of aromatic polyimide resins, and in addition, some characteristics of the aliphatic polyimide resins are not very satisfactory in comparison with aromatic polyamideimide resins. For example, when these resins are used as insulating varnishes, the dielectric breakdown voltage of each varnish changes as follows, after heat-treatment for 168 hours at 260.degree. C. In the case of aromatic polyimides and aromatic polyamideimides, the residual value is 90% or more of the initial value, but in the case of polyimides of BTC, the residual value is 70 to 80%. On the other hand, Japanese Patent Publication No. 1797/74, for example, suggests that heat resistant polymers are prepared by reaction of BTC and a tricarboxylic acid anhydride derivative such as phenyl trimellitic anhydride with a diamine, and in preparation of these polymers, solvents of relatively strong polarity such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone or dimethylsulfoxide are essentially used. However, the use of these solvents is not desirable in that an inflammation of the human body results when these solvents contact the body and also in that these solvents have a strong odor and, therefore, it is advisable to avoid the use of these solvents, if possible, and also, particularly, from the standpoint of prevention of air pollution.